ANTI-THYMOCYTE GLOBULIN TREATMENT OF A PATIENT FOR PAROXYSMAL-NOCTURNAL HEMOGLOBINURIA-APLASTIC ANEMIA SYNDROME - COMPLEMENT ACTIVATION ANDTRANSIENT DECREASE OF THE PNH CLONE

Paroxysmal nocturnal haemoglominuria (PNH) is an acquired clonal stem cell disorder resulting in insufficient and defective haematopoesis as sociated frequently with aplastic anaemia (AA). A deficiency of the gl ycosyl phosphatidylinositol (GPI)-anchored complement activation regul atory proteins CD55 and CD59 is responsible for an increased sensitivi ty of erythrocytes to complement attack leading to chronic intravascul ar haemolysis with haemoglobinuria. In this study we investigated the effects of complement activation caused by anti-thymocyte globulin (AT G) treatment on the PNH clone in a patient affected with the PNH/AA-sy ndrome. Fluid phase complement components C3, C4, C6 and terminal comp lement complex (TCC) were assayed by ELISA. CD55, CD59 and cell-associ ated TCC were monitored by flow cytometry. ATG treatment resulted in p rofound systemic complement activation which led to a decrease in the levels of native C3 and C4 to 65% and 40%, respectively, of the origin al levels on day 5 and of C6 and TCC to 61% and 23%, respectively, on day 10. A return to pre-treatment levels was observed for C3 by day 15 , for C6 by day 30 and for C4 by day 90. Flow cytometry revealed that the deficiency in the GPI-anchored protein was restricted to granulocy tes, while lymphocytes remained unaffected. Cell-bound TCC increased b y 1.67-fold and 2.37-fold on day 5 and day 10, respectively, decreasin g to 1.40-fold and 1.30-fold on day 15 and day 30, respectively. The p ercentage of PNH granulocytes as identified by the absence of the CD55 - and CD59-antigens exhibited a temporary decrease from 72% on day 0 t o 65% on day 5 and 59% on day 10 and returned thereafter to the origin al percentage of 70% by day 15 and exceeding this level to 76% on day 30 and 79% on day 90. We report profound activation of the classical p athway of the complement cascade and the terminal complement complex b y the globulin leading to a transient decrease of the PNH clone, presu mably due to subsequent lysis of the PNH cells devoid of complement re gulatory proteins.